The street names can be an issue. Each city and state may frequently place the common name of the street on signs. The City Nav pulls the data from sources that only list the official street numbers. So that could be an issue for what you are trying to do.
When you say the distances are not accurate are you saying the unit does not route you the best way possible or the straight line distance is not correct?
If you like the 705 but want the DVD version maps instead (my personal preference) then you could return the one with the maps on SD card and buy the version with no maps and get the DVD seperatly.
The 500 will download courses from the web. Are you familiar with the difference between a route and a course?